He didn't cut weight, he was dehydrated from an excess of training, he had 3 guys fighting that weekend on different cards (he's a coach) and was in the gym with them. If he's to be believed he was exhausted and lacking water for at least the 3 days prior to the fight and was really struggling with pains in his knee (which has always bothered him since tweaking it in his first fight with Andy). Unlikely Fedor was on anything for his fight Hendo, he and Hendo were tested by the same athletic commission and as little value as it might hold, I personally don't think Fedor is the type of man to cheat. I don't think he saw any value in it. Could I be wrong? Sure, but it's the vibe I get from him. But like I said he was a LHW coming into the fight with Fedor and had previously been training for LHW fights. Even prior to being booked to fight Fedor he was in regular camp for LHW fights, as he expected to defend his title when he was approached with the offer to fight an MMA legend that he also respected.
It's already been mentioned but he said himself that the weight cut to 185 for the Shields' fight was one of his worst and it affected his cardio for that fight. Believable as Hendo has never shown consistent issues with cardio on the level that he showed in that fight.